1. Field of the Invention
The invention relates to a signal processing circuit substrate used for a liquid crystal display unit, and more particularly to such a signal processing circuit substrate on which a device such as a resistor is mounted.
2. Description of the Related Art
There has been known a signal processing circuit substrate on which a variable resistor is mounted for optimally adjusting a supplied voltage. For instance, such a signal processing circuit substrate is equipped in a liquid crystal panel constituting a liquid crystal display unit used as a display screen of a personal computer.
FIGS. 1A and 1B illustrate a liquid crystal display panel unit including a conventional signal processing circuit substrate. FIG. 1A includes a plan view, a bottom view and a side view of the liquid crystal display panel unit, and FIG. 1B is a cross-sectional view taken along the line 1B—1B in FIG. 1A.
As illustrated in FIGS. 1A and 1B, a liquid crystal panel unit 1 has an upper surface la and a lower surface 1b. A rectangular liquid crystal panel 2 and a frame-shaped shield member 3 covering an outer edge of the liquid crystal panel 2 are arranged on or above the upper surface 1a. First and second signal processing circuit substrates 4a and 4b are arranged on the lower surface 1b along longer and shorter sides, respectively.
As illustrated in FIG. 1B, the shield 3 is in the form of a frame having a reverse L-shaped cross-section. The shield 3 covers an entire outer edge of the liquid crystal panel 2, and defines an outer wall 3a as a sidewall of the liquid crystal panel unit 1. As illustrated in FIG. 1A, the shield 3 is formed with a through-hole 5a. As illustrated in FIG. 1B, the through-hole 5a is open also to the wall 3a. 
The first signal processing circuit substrate 4a has an extending portion 4A in alignment with the through-hole 5a. 
A light-guide member 7 having a multi-layered structure is formed beneath a lower surface of the liquid crystal panel 2, as illustrated in FIG. 1B. The liquid crystal panel 2 together with the light-guide member 7 is assembled into a frame-shaped back light chassis. A light diffusion sheet 7a entirely covers the light-guide member 7 therewith at a lower surface of the light-guide member 7. The back-light chassis 8 has such a size that an outer edge of the liquid crystal panel 2 is located below the shield 3.
There is a space between an outer wall of the back-light chassis 8 and the wall 3a of the shield 3 below the shield 3 through which the through-hole 5a is formed. A variable resistor 6a used for optimally adjusting a supplied voltage is mounted on the extending portion 4A in the space, as illustrated in FIG. 1B. The variable resistor 6a has a value adjustment portion 6b through which a resistance of the variable resistor 6a can be adjusted. The variable resistor 6a is positioned in the space such that the value adjustment portion 6b faces the through-hole 5a. 
Hence, a resistance of the variable resistor 6a can be adjusted by inserting an adjuster such as a screwdriver into the space through the through-hole 5a, and rotating the value adjustment portion 6b by means of the adjuster. Thus, a supplied voltage can be optimized at the side of the first surface 1a of the liquid crystal panel unit 1.
It is necessary to adjust a resistance of the variable resistor 6a in every liquid crystal panel unit 1 such that a supplied voltage is an optimal voltage when a direct current is turned into an alternating current in a liquid crystal display unit driven with an alternating current. If the resistance is not properly adjusted, there is likely to occur flicker, burning or sticking, and non-uniformity in display.
FIGS. 2A to 2C illustrate a liquid crystal panel unit including another conventional signal processing circuit substrate. FIG. 2A includes a front view, a bottom view and a side view of the liquid crystal panel unit, FIG. 2B is a cross-sectional view taken along the line 2B—2B in FIG. 2A, and FIG. 2C is an enlarged view of a portion of the liquid crystal panel unit.
As illustrated in FIG. 2B, a liquid crystal panel unit 9 includes a variable resistor 6d which is mounted on a first signal processing circuit substrate 4c, in place of the variable resistor 6a. The variable resistor 6d includes the value adjustment portion 6b on a top thereof and an additional value adjustment portion 6c at a bottom thereof.
The shield 3 is not formed with a through-hole such as the through-hole 5a, but the first signal processing circuit substrate 4c is formed with a through-hole 5b. The variable resistor 6d is mounted on the first signal processing circuit substrate 4c such that the additional value adjustment portion 6c is located within the through-hole 5b. 
The other structures of the liquid crystal panel unit 9 are the same as the structures of the liquid crystal panel unit 1 illustrated in FIGS. 1A and 1B.
Hence, a resistance of the variable resistor 6d can be adjusted by inserting an adjuster such as a screwdriver into the space through the through-hole 5b, and rotating the value adjustment portion 6c by means of the adjuster. Thus, a supplied voltage can be optimized at the side of a second surface 9b of the liquid crystal panel unit 9.
The above-mentioned liquid crystal panel units 1 and 9 are designed to have a small frame and a small thickness in order to be thin and have a broad display area in response to requirements of a small size, a light weight and small power consumption in a personal computer. For instance, the liquid crystal display units 1 and 9 are designed to have a thickness in the range of 6 to 8 mm, and have an effective pixel area occupying about 90% of an area of the panel.
In addition, the liquid crystal panel units 1 and 9 have to be designed such that the value adjustment portions 6b and 6c are exposed through the through-holes 5a and 5b, respectively, because the variable resistors 6a and 6d are adjusted with an operator observing a display screen after the first signal processing circuit substrates 4a and 4c have been mounted on the liquid crystal panel 2.
The variable resistor 6a which is adjusted at the side of the first surface 1a of the liquid crystal panel unit 1 has a height of about 1 mm, and is lower than the variable resistor 6d which is adjusted at the side of the second surface 9a of the liquid crystal panel unit 9 and has a height of about 1.5 mm. Accordingly, the variable resistor 6a is more appropriate for reduction in a height of a liquid crystal panel unit. However, as illustrated in FIG. 1B, it is necessary to form the through-hole 5a throughout the shield 3.
In the liquid crystal display panel 9, a space between the liquid crystal panel 2 and the light-guide member 7 is smaller than 1.5 mm at a central area of the first signal processing circuit substrate 4c. Hence, the variable resistor 6c cannot be positioned in the central area of the first signal processing circuit substrate 4c, and accordingly, has to be positioned in a marginal area of the first signal processing circuit substrate 4c in which a sufficiently broad space can be ensured.
In addition, it is unavoidable that the shield 3 formed with the through-hole 5a would have a reduced mechanical strength.
Furthermore, if the variable resistor is mounted on the signal processing circuit substrate in a marginal area thereof, the back-light chassis 8 has to be formed thin in order to ensure a sufficient space for mounting the variable resistor therein in a limited space inside the shield 3. As a result, it is unavoidable that a mechanical strength of the back-light chassis 8 is reduced. This is quite severe condition for the shield 3 which is formed as narrow as possible in order to form an effective pixel area as wide as possible.
A space between the first signal processing circuit substrate 4a, 4c and the light guide 7 is almost in its limit. If the space were formed greater, the liquid crystal panel 2 has to be formed thinner, resulting in that it would be impossible to ensure requisite functions of the liquid crystal panel 2. Hence, it is quite difficult to form the space greater.
Since the value adjustment portion 6c of the variable resistor 6d is exposed through the through-hole 5b formed through the signal processing circuit substrate 4c, an adjuster such as a screwdriver could be readily engaged to the value adjustment portion 6c. On the other hand, since the value adjustment portion 6b of the variable resistor 6a is much spaced away from the through-hole 5a, an operator has to look for the value adjustment portion 6b with an adjuster after inserting the adjuster through the through-hole 5a. As a result, an operator may damage the variable resistor 6a with the adjuster.
Thus, in the above-mentioned conventional liquid crystal panel units 1 and 9, the variable resistors 6a and 6d have to be positioned in a limited area, and the variable resistors 6a and 6d may be damaged in assembling the liquid crystal panel units 1 and 9, due to reduction in a mechanical strength.
In the above-mentioned liquid crystal panel units 1 and 9, the variable resistors 6a and 6d are explained as examples, but the same problems as mentioned above are caused when other devices such as a capacitor are to be mounted on a signal processing circuit substrate.